The invention relates to a process for the production of aluminium hydroxide by digesting bauxite with an alkali solution and precipitating aluminium hydroxide from the digestion solution.
The most common method for the production of aluminium hydroxide from alumina-containing ores, hereafter referred to as bauxite, is the Bayer process. The aluminium hydroxide produced is usually converted into alumina intended for the electrolytical production of aluminium.
In the Bayer process bauxite is digested with an aqueous alkali solution at elevated temperature. Spent liquor, obtained after precipitating aluminium hydroxide from the digestion solution in a later stage in the Bayer process, is used as aqueous alkali solution. After the digestion step the obtained slurry, comprising a solution of alkali aluminate in which solution the constituents of the bauxite which are insoluble in the alkali solution are suspended, is cooled. Commonly flash cooling is used. The flash steam thus produced is used for (pre)heating bauxite and spent liquor before digestion. After separation and washing of the solid phase, generally referred to as red mud, the supersaturated sodium aluminate solution is further cooled and seeded with aluminium hydroxide particles to cause precipitation of aluminium hydroxide. The precipitated aluminium hydroxide is separated, washed, dried and calcined at high temperatures to form alumina. The sodium aluminate liquor with its reduced alumina content (spent liquor) is recycled to the bauxite digestion step after concentrating, if necessary, to remove water which has been introduced into the system, for example during washing the red mud and/or the precipitated aluminium hydroxide, and recharged with sodium hydroxide to make-up for sodium hydroxide losses from the circuit.
The alumina in bauxite generally exists in the form of alumina trihydrate (Al.sub.2 O.sub.3.3H.sub.2 O or Al(OH).sub.3, gibbsite) and/or alumina monohydrate (Al.sub.2 O.sub.3.H.sub.2 O or AlO(OH), boehmite, diaspore). Generally, alumina trihydrate is the main alumina component. The difference in solubility between the trihydrate and the monohydrates in aqueous alkali solutions requires different conditions in the digestion process.
Alumina monohydrate is not as easily dissolved as alumina trihydrate. A higher caustic concentration and/or a higher digestion temperature and/or a lower alumina to caustic digestion ratio is required to dissolve substantially all the alumina monohydrate from the bauxite. Usually a higher digestion temperature is used to digest the monohydrate.
The bauxite for the Bayer process is usually ground before the extraction procedure. The grinding process is often a wet grinding process in which part of the spent liquor (usually between 5 and 25 per cent) is used. The other part of the spent liquor is directly used for the digestion step.
The digestion process may be classified on the basis of the preheating procedure into a one-stream process, in which the bauxite and the aqueous alkali solution necessary for the extraction are mixed together, whereafter the mixture is subjected to preheating and extraction, and a two-stream process, in which a slurry of the bauxite in a part of the aqueous alkali solution, which slurry is usually obtained in the above-mentioned wet grinding process, and the remainder of the aqueous alkali solution are preheated individually, mixed together and then subjected to extraction.
A disadvantage of the one-stream process is the severe erosion and/or scaling which may occur during the indirect heating of the caustic bauxite slurry in indirect heat exchangers. This holds especially when the digestion step has to be carried out at relatively high temperatures because of the presence of alumina monohydrate in the bauxite feed. This problem may be solved by using steam injection in direct contact heaters. However, if the entire liquor stream is heated in this manner, excessive dilution will result.
A disadvantage of the two-stream process is the severe corrosion, which may occur in the indirect heaters used for heating the spent liquor, especially when the free caustic concentration is high and/or the temperature is high. The use in the two-stream process of indirect heat exchangers manufactured from a special material which will not be attacked by the hot alkali solution is less desirable in view of the high costs.
When bauxite containing an economically winnable amount of alumina monohydrate is used in the Bayer process, it is common practice that at least part of the digestion step is carried out at high temperature, and thus also at high pressure. Two processes are in use for extracting alumina from such bauxites. In the first process the digestion of both mono- and trihydrate is carried out under digestion conditions suitable for monohydrate-containing ores. In the second process the trihydrate is first digested from the bauxite under digestion conditions suitable for trihydrate-containing ores and the resulting residue is then treated under digestion conditions suitable for monohydrate-containing ores.
In the first process the bauxite is treated in a single digestion step at severe conditions of high temperature and high pressure suitable for digestion of the monohydrate. Consequently, the capital costs of such a plant are high.
In the second process the bauxite is first digested at conditions suitable for trihydrate extraction. In the second step the residue, which includes the monohydrate, is re-digested with a second portion of spent liquor under conditions suitable for monohydrate-containing ores.
Disadvantages of these processes are the severe erosion and/or scaling which may occur in case a one stream process is used or the severe corrosion which may occur in the case that a two-stream process is used at high temperature and/or using a high free caustic concentration.
The process of the present invention especially relates to the digestion at relatively high temperatures, usually between 210.degree. and 270.degree. C., of bauxite containing alumina trihydrate together with a certain amount of alumina monohydrate.